微机电系统中的微观黏滑、黏附与控制

微观黏滑和黏附失效是微机电系统中的常见现象,该现象主要是由于受包括静电力、范德华力及毛细力等各种表面力所起的主导作用而产生的．采用黏着接触理论和运动分析方法,得到了微观摩擦试验中黏滑出现的无量纲黏滑数,表微观黏滑是接触表面特性、形貌参数、接触载荷及滑动速度等综合作用的结果,进而获得黏滑现象的各种临界参数,提出了黏滑行为控制的表面修饰与形貌设计依据;针对微构件的黏附失效,采用Laplace公式并结合微构件的变形分析,探究了毛细力作用下微构件的变形特征与失稳行为,发现其变形过程中存在着不稳定的临界点,对应黏附行为的发生,进而提出了微构件防黏附的结构设计．     

织构表面在油和水中的润湿性及摩擦学性能

为研究织构形貌对表面润湿性和摩擦学性能的影响,建立凹坑表面流体动压润滑数学模型,计算底面为正方形的棱柱和圆台形凹坑表面的润滑膜动压承载力。计算结果表明,凹坑面积率为19.6%时,圆台形凹坑表面的流体动压润滑膜承载力是棱柱凹坑表面的2.4倍。利用激光加工技术,在5083船用铝合金表面加工与数学模型一致的棱柱形和圆台形凹坑织构,利用低表面能修饰和溶胶凝胶法涂敷SiO_2改变表面润湿性能。接触角测试显示,棱柱形凹坑表面的接触角比圆台形凹坑表面大2°~4°。摩擦实验显示,将织构和化学组分相结合的双疏表面可以显著地提高摩擦学性能。圆台形凹坑表面的摩擦学性能优于棱柱形凹坑表面,与计算结果相符。凹坑形貌对表面摩擦学性能的影响大于对表面润湿性的影响。     

铜合金压制凹坑织构表面的磨损性能

为了改善铜合金材料表面的摩擦磨损性能,采用压制加工方法,在CuSn6锡青铜试件表面制备出凹坑织构,分别进行压制凹坑织构试件和无织构试件的摩擦磨损试验,测量了摩擦系数和磨损量,观察了磨损表面及磨屑,分析了压制凹坑织构对试件磨损性能的影响。结果表明:与无织构试件相比,压制凹坑织构的磨损量减少。无织构试件的磨损机制为磨粒磨损与黏着磨损,压制凹坑织构的磨损机制为磨粒磨损和少量黏着磨损。压制凹坑边缘存在硬化区域,提高了试件表面平均硬度。凹坑容屑能力与边缘硬化区域有助于改善压制凹坑试件磨损性能。     

凹坑体积对中碳钢织构表面摩擦磨损特性的影响

为了改善高速钢表面的摩擦磨损性能,采用钻削加工的方法,在45钢表面加工出不同深度的凹坑织构,进行凹坑织构表面与GCr15的摩擦磨损试验,测量了摩擦系数和磨损量,观察了试件磨痕表面,分析了凹坑体积对织构表面摩擦磨损性能的影响。结果表明:凹坑织构表面的摩擦系数和磨损量首先随着凹坑体积的增加而减小,当凹坑体积达到某一值后,摩擦系数和磨损量不再随着体积的增加而减小;凹坑织构的容屑作用有助于提高织构表面的摩擦磨损特性; 45钢在磨损过程中存在磨屑的搭桥覆盖现象,影响了凹坑的容屑能力,限制了凹坑织构表面耐磨性能的提高。     

TC4合金微动磨损部分滑移区摩擦参数的演变

选取三种硬度对偶件(HV680GCr15、HV1500Si3N14和HV2300Al2O3)为变量,研究TC4合金在室温和高温下微动磨损部分滑移区摩擦特性参数的演变.试验结果表明,在三种硬度对偶件配合下,TC4合金在室温和高温下的微动磨损均处于典型部分滑移区,未发生向其他两区域的转变.对偶件硬度对部分滑移区摩擦特性参数的影响较为显著:对偶件硬度对表面损伤的影响最为突出,磨痕表面由清晰可辨的黏着与微滑两区逐渐转变为黏着、微滑以及严重塑性变形的环状过渡区,表面轮廓由凹凸峰曲线转变为中心明显凹坑、边缘较为粗糙的规则"U"型斑;对偶件硬度对损伤机制的影响较小,由黏着磨损逐渐过渡到氧化伴随黏着磨损;对偶件硬度对摩擦系数几乎不产生影响,曲线变化规律,稳定值基本相同.     

强噪声作用下复合材料壁板气动弹性摩擦非线性振动

基于von Karman非线性应变、Reddy三阶剪切锯齿理论和准稳态一阶活塞理论建立了强噪声载荷作用下含摩擦边界复合材料壁板的气动弹性非线性动力学有限元数值计算模型,采用高斯限带白噪声和宏观黏滑摩擦模型分别来描述噪声载荷和层合壁板边界的非光滑摩擦力特性,应用时域直接积分Newmark-β方法结合Newton-Raphson迭代法求解了声载荷和气动载荷联合作用下壁板的非线性振动响应,研究了声载荷和摩擦边界对壁板颤振特性的影响。结果表明,摩擦边界上的滑移运动可耗散层合壁板的振动能量,对壁板的振动响应产生抑制作用;在强噪声和气动载荷作用下,壁板的振动会呈现出由气动载荷主导下的极限环振动和强噪声载荷主导下近似服从正态分布的随机振动。     

12Cr13钢表面双辉等离子沉积SiC涂层的形貌和耐磨性能

目前对不锈钢表面SiC涂层的摩擦磨损性能与制备过程中H2流量的相关性研究较少.通过双辉等离子表面冶金技术在传动机械用12Cr13不锈钢基表面制备SiC涂层,利用表面形貌表征和摩擦磨损试验,分析了H2流量对SiC涂层的表面形貌和耐磨性能的影响.结果表明:SiC涂层的表面呈现出环状与片层状花纹及凹坑、颗粒等形貌,并且随着H2流量的增加,其表面形貌缺陷程度也不断增加,各衍射峰呈现明显的增强或者减弱趋势.12Cr13不锈钢基体磨损测试后表现出明显的磨粒磨损特点;随着H2流量的增加,涂层与接触副接触时容易发生脆性破裂,涂层磨损明显,H2流量达到20 mL/min时,磨痕表现为深犁沟形态,此时以黏着磨损机制为主.     

不同速度、载荷下点接触弹流润滑计算

人造织构表面在润滑条件下,较光滑表面提高了润滑性能,减少磨损.为了深入了解织构表面形貌对流体润滑影响的机理,本文提出了规则凹坑表面油膜压力的计算方案和公式,比较了粗糙表面与光滑表面的最小油膜厚度和压力分布.结果表明,低速、低载时,规则凹坑表面具有更大的最小油膜厚度,更接近弹流润滑的典型膜厚,而接触面间的最大油膜压力降低.     

玻璃纤维增强聚醚醚酮复合材料在水润滑下的摩擦学性能

利用球盘式摩擦磨损试验机考察了玻璃纤维(GF)增强聚醚醚酮(PEEK)复合材料在干摩擦和水润滑条件下的摩擦磨损性能,并探讨了其磨损机理。结果表明:在干摩擦和水润滑条件下,PEEK和GF/PEEK的摩擦因数和磨损率均随载荷和对磨时间的增加逐渐增大并趋于稳定,GF的加入可以显著降低GF/PEEK复合材料的摩擦因数和磨损率;在水润滑条件下,PEEK和GF/PEEK的摩擦因数和磨损率比干摩擦下显著降低。干摩擦下,PEEK以黏着磨损和磨粒磨损的混合磨损形式为主,水润滑条件下,磨损方式主要是以轻微的黏着磨损为主;干摩擦下,GF/PEEK磨损表面有大量的微观断裂裂纹和破碎,以磨粒磨损和疲劳磨损为主,水润滑条件下,磨损表面仅有微观切削的痕迹,磨损方式以轻微磨粒磨损为主。由于水的冷却和润滑作用,使得复合材料向对偶钢球的黏着转移明显减弱,同时阻止了对偶钢球上的Fe向复合材料磨损表面转移,从而减轻摩擦、降低摩擦表面温升,显著改善复合材料的摩擦磨损性能。     

抑制油气井钻柱黏滑振动控制器设计与应用

油气井钻进过程中,钻柱系统深入地下几千米,容易引发钻柱黏滑振动现象,进而损坏钻井设备、降低钻进效率及危害钻井安全。为了抑制钻柱黏滑振动,提高系统抗干扰能力,建立了钻柱双自由度集中参数模型及非线性摩擦扰动模型,设计一种状态观测器,提出了速度补偿反馈控制方案。在仿真系统中验证了控制方案的可行性;仿真结果表明,设计的控制器可以使波动的钻头转速逐步收敛于输入参考值,并且闭环控制系统表现出良好的动态性能;基于速度补偿反馈控制器设计了钻柱黏滑振动控制系统,并在钻进现场完成测试实验。实验表明,设计的控制系统可以有效地抑制钻柱黏滑振动,从而进一步提高钻进效率、保障钻进安全。     

Adhesion forces measured by atomic force microscopy in humid air

Adhesive forces measured with an atomic force microscope under ambient conditions are generally regarded to be dominated by non-surface-specific capillary force. In this study, the nature of the "pull-off" force on a variety of surfaces was investigated as a function of relative humidity. The results indicate that even under the condition where capillary condensation occurs there is chemical specificity in the measured pull-off force. Issues such as tip-surface contact time and surface roughness were ruled out as possible artifacts. A mathematical model of pull-off force as a function of relative humidity is proposed in which the chemical specificity is explained.     

Shearing of fibrillar adhesive microstructure: friction and shear-related changes in pull-off force.

To characterize the effect of shearing on function of fibrillar adhesive microstructure, friction and shear-related changes in pull-off force of a biomimetic polyvinylsiloxane mushroom-shaped fibrillar adhesive microstructure were studied. In contrast to a control flat surface, which exhibited pronounced stick-slip motion accompanied with high friction, the fibrillar microstructure demonstrated a stable and smooth sliding with a friction coefficient approximately four times lower. The structured contact also manifested zero pull-off force in a sheared state, while the flat surface exhibited highly scattered and unreliable pull-off force when affected by contact shearing. It appears that the fibrillar microstructure can be used in applications where a total attachment force should be generated in a binary on/off state and, most surprisingly, is suitable to stabilize and minimize elastomer friction.     

The adhesion model considering capillarity for gecko attachment system.

Geckos make use of approximately a million microscale hairs (setae) that branch off into hundreds of nanoscale spatulae to cling to different smooth and rough surfaces and detach at will. This hierarchical surface construction gives the gecko the adaptability to create a large real area of contact with surfaces. It is known that van der Waals force is the primary mechanism used to adhere to surfaces, and capillary force is a secondary effect that can further increase adhesive force. To investigate the effects of capillarity on gecko adhesion, we considered the capillary force as well as the solid-to-solid interaction. The capillary force expressed in terms of elliptical integral is calculated by numerical method to cope with surfaces with a wide range of contact angles. The adhesion forces exerted by a single gecko spatula in contact with planes with different contact angles for various relative humidities are calculated, and the contributions of capillary force to total adhesion force are evaluated. The simulation results are compared with experimental data. Finally, using the three-level hierarchical model recently developed to simulate a gecko seta contacting with random rough surface, the effect of the relative humidity and the hydrophobicity of surface on the gecko adhesion is investigated.     

Contact mechanics modeling of pull-off measurements: effect of solvent,probe radius,and chemical binding probability on the detection of single-bond rupture forces by atomic force microscopy

Pull-off forces for chemically modified atomic force microscopy tips in contact with flat substrates coated with receptor molecules are calculated using a Johnson, Kendall, and Roberts contact mechanics model. The expression for the work of adhesion is modified to account for the formation of discrete numbers of chemical bonds (nBonds) between the tip and substrate. The model predicts that the pull-off force scales as nBonds(1/2), which differs from a common assumption that the pull-off force scales linearly with nBonds. Periodic peak progressions are observed in histograms generated from hundreds of computed pull-off forces. The histogram periodicity is the signature of discrete chemical interactions between the tip and substrate and allows estimation of single-bond rupture forces. The effects of solvent, probe tip radius, and chemical binding probability on the detection of single-bond forces are examined systematically. A dimensionless parameter, the effective force resolution, is introduced that serves as a quantitative predictor for determining when periodicity in force histograms can occur. The output of model is compared to recent experimental results involving tips and substrates modified with self-assembled monolayers. An advantage of this contact mechanics approach is that it allows straightforward estimation of solvent effects on pull-off forces.     

多壁碳纳米管／硅橡胶复合材料的湿度敏感特性

采用多壁碳纳米管和硅橡胶制备多壁碳纳米管／硅橡胶复合材料。研究了该复合材料的导电网络和湿敏特性。通过透射电镜观察经过化学修饰和未修饰的多壁碳纳米管的表面形貌,扫描电镜观察多壁碳纳米管／硅橡胶复合材料形成的导电网络结构;分析该复合材料的导电机理,研究多壁碳纳米管／硅橡胶感湿特性。结果表明,相对湿度在11％-98％的范围,...     

Nanogeometry matters: unexpected decrease of capillary adhesion forces with increasing relative humidity

The sticking effect between hydrophilic surfaces occurring at increasing relative humidity (RH) is an everyday phenomenon with uncountable implications. Here experimental evidence is presented for a counterintuitive monotonous decrease of the capillary adhesion forces between hydrophilic surfaces with increasing RH for the whole humidity range. It is shown that this unexpected result is related to the actual shape of the asperity at the nanometer scale: a model based on macroscopic thermodynamics predicts this decrease in the adhesion force for a sharp object ending in an almost flat nanometer-sized apex, in full agreement with experiments. This anomalous decrease is due to the fact that a significant growth of the liquid meniscus formed at the contact region with increasing humidity is hindered for this geometry. These results are relevant in the analysis of the dynamical behavior of nanomenisci. They could also have an outstanding value in technological applications, since the undesirable sticking effect between surfaces occurring at increasing RH could be avoided by controlling the shape of the surface asperities at the nanometric scale.     

From Single Particle AFM Studies of Adhesion and Friction to Bulk Flow: Forging the Links

The atomic force microscope (AFM) has been used to study inter-particle contacts in air for a range of model particles and cohesive granular materials of commercial importance. Adhesion (or pull-off force), friction and its load dependence, and particle size, morphology and roughness were measured for glass ballotini, fumed silica, alumina, limestone, titania and zeolite. Particle-wall contacts and effects of relative humidity were also studied. Most of the results, after allowing for roughness, are consistent with JKR contact mechanics and capillary bridge theory; however, the main object of the present work is to demonstrate semi-quantitative links between the AFM measurements and related bulk flow and cohesion measurements performed in parallel on the same materials. A simple model of a particle assembly will be used to compare average contact forces in typical single-particle AFM experiments and typical bulk experiments, and thus identify those regimes of powder flow where the two approaches overlap, and AFM measurements may be used with some confidence in more sophisticated modeling based on distinct element analysis (DEA). Four areas will be discussed briefly: (1) The apparent analogy between bulk yield loci and single-particle friction-load data; (2) Cohesion data and particle size effects; (3) Bulk tensile strength and single particle pull-off force; (4) Bulk wall friction and single-particle-wall friction. It is found that typical single-particle AFM experiments and bulk shear experiments converge for small particles (～ 4 μm) and low consolidation stress, when the average inter-particle contact forces are of the order 20–100nN, involve single or few asperities, and are not much larger than pull-off forces. For large particles and high consolidation loads the data do not overlap and AFM measurements may be less useful as input to simulations where sliding friction is less important, and where large normal contact forces dominate over tangential forces and are responsible for the shear strength.     

Nano-tribological characteristics of TiO<Subscript>2</Subscript> films on 3-mercaptopropyl trimethoxysilane sulfonated self-assembled monolayer

Silane coupling reagent (3-mercaptopropyl trimethoxysilane (MPTS)) was used to prepare twodimensional self-assembled monolayer (SAM) on silicon substrate. The terminal -SH group was in situ oxidized to −SO₃H group to endow the film with good chemisorption ability. Then TiO₂ thin films were deposited on the oxidized MPTS-SAM to form composite thin films, making use of the chemisorption ability of the −SO₃H group. Atomic force microscope (AFM) and contact angle measurements were used to characterize TiO₂ films. Adhesive force and friction force of TiO₂ thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. Results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. In order to study the effect of capillary force, tests were performed in various relative humidities. Results showed that the adhesive force of silicon substrate increases with relative humidities and the adhesive force of TiO₂ thin films only increases slightly with relative humidity. Research showed that surfaces with more hydrophobic property revealed the lower adhesive and friction forces.     

Parallel-local anodic oxidation of silicon surfaces by soft stamps

We investigate the fabrication of nanometric patterns on silicon surfaces by using the parallel-local anodic oxidation technique with soft stamps. This method yields silicon oxide nanostructures 15?nm high, namely at least five times higher than the nanostructures made with local anodic oxidation using atomic force microscopy, and thanks to the size of the stamp enables one to pattern the surface across a centimetre length scale. To implement this technique, we built a machine to bring the metallized polydimethylsiloxane stamp in contact with the silicon surface, subsequently inserted in a sealed chamber with controlled relative humidity. The oxide nanostructures are fabricated when a bias voltage of 36?V is applied between the stamp and the silicon for 2?min, with a relative humidity of 90%. The flexibility of the stamp enables a homogeneous conformal contact with the silicon surface, resulting in an excellent reproducibility of the process. Moreover, by means of two subsequent oxidations with the same stamp and just rotating the sample, we are able to fabricate complex nanostructures. Finally, a detailed study of the oxidation mechanism, also using a finite element analysis, has been performed to understand the underlying mechanism.